This invention relates to an improvement in the use of sodium silicate as a binder for sand molds and cores used in foundries to cast various metals. More particularly the improvement involves a sodium-lithium binder for the sand cured in a microwave oven.
Sodium silicate has long been known as a binder for sand molds and cores used in metal casting. British Pat. No. 15,619 issued in 1898 discloses sodium silicate set with CO.sub.2 as a suitable binder for molds and cores. Even though this process has been known and practiced for more than 75 years, it is not completely satisfactory. Uniform administration of CO.sub.2 to the silicate-saturated sand is difficult to achieve, resulting in incompletely set molds that must be rejected. Removal of cores from the finished metal castings is also a long-standing problem. In addition to these problems, molds and cores do not maintain the required strength upon storage in humid conditions.
These problems are well recognized, and other means of augmenting sodium silicate binders have been proposed. U.S. Pat. Nos. 2,748,435; 3,135,029 and 3,209,420 disclose binder systems with silicate and thermosetting resins, acrylamide polymers and urea formaldehyde resins, respectively. These patents, among numerous others, attempt to solve not only the shortcomings of the silicate bound systems, but those of the organic binder systems. None of these approaches appears to be universally accepted.
An additional, and more recent, problem with the use of sodium silicate binders is associated with reuse of the sand to make further molds or cases. Reuse of the sand from silicate-bound cores suffers from the silicate gel that surrounds the sand grains. This situation manifests in short setting times which do not allow proper working. This problem, as well as others, was addressed by Cole, Owusu and Nowichi in a publication, "Microwave Cured Sodium Silicate Bonded Cores," Trans Amer. Foundrymen's Soc., 87, 605-612, 1979. In this system less sodium silicate was required to provide the bonding, and therefore the sand could possibly be reused more readily. However, this approach did not provide any relief from the problem of moisture-induced strength loss.
The ideal binder system for foundry molds using a silicate solution should have the following characteristics: the amount of silicate should be low so that sintering is minimized, shakeout after casting is facilitated, and the sand can be reclaimed more readily. The sand-binder mixture should set quickly while providing sufficient green strength so that the piece can be handled. This strength should be persistent so that the mold or core can be stored under ambient conditions. The binder should not release noxious vapors at practical metal-pouring temperatures, and it should be economical.
We have found that a sodium silicate solution combined with lithium ion and the mold sand can be cured in a microwave oven to provide these characteristics. In searching the art area we have not found references to such binders. The closest reference to a silicate binder system containing both sodium and lithium ions appears to be U.S. Pat. No. 3,180,746 to Patton and Cox. This patent discloses a surface bonding agent employed in preparing coatings such as zinc-rich corrosion resistant paints and the like. The process for preparing the so-called lithium-sodium-silicate binder solution involves autoclaving a mixture of silica gel, sodium hydroxide, lithium hydroxide and water. The temperature is raised to 150.degree. C. in a period of 30 to 90 minutes. Cooling from 150.degree. C. to 95.degree. C. must also be accomplished in this 30 to 90 minute period. The temperature must now be controlled so that the temperature declines from 95.degree. C. to 40.degree. C. in 150 to 240 minutes. Cooling to room temperature requires 60 to 120 minutes. Even if this binder were suitable for sand foundry molds, its preparation is too complicated and expensive.